The present invention relates generally to multi-layer ceramic (MLC) substrates and, more specifically, to conductive line features and methods for printing such features to enhance reliability of MLC substrates.
Multi-layer ceramic (MLC) substrates, such as are used for chip carriers, comprise a number of discrete ceramic sheets laminated and sintered together. Each sheet has conductive lines printed on its surface with a conductive substance such as a metal, for example, copper paste. Holes punched through the ceramic sheet before sintering of the sheet (the unsintered sheet is known in the art as a xe2x80x9cgreenxe2x80x9d sheet) are filled with the conductive paste to provide conductive connections or xe2x80x9cviasxe2x80x9d between layers of the substrate. Thus, for example, a chip mounted on a completed MLC substrate is electrically connected to an underlying circuit board through the lines and vias of the MLC. Reliable connections in the MLC are critical to achieving the expected performance of the chip; the reliability of the connections is highly dependent upon the integrity of the line-to-via junctions.
Typically, the manufacture of such MLC substrates starts with a glass ceramic powder that is formed into a slurry and cast into a large sheet similar to how a sheet of paper is made. The large cast sheet is then dried and cut into smaller blanks. Via holes are punched into the blanks, and the lines are screen printed onto the blanks using masks and a conductive paste. Then, the layers of the MLC are stacked together and laminated in a press, cut to size, and sintered in an oven to create a homogenous ceramic substrate. One or more terminal plating steps may then be performed, including attaching one or more layers of thin film interconnects on top of the substrate, before joining the chip to the substrate.
Referring now to FIG. 1, there is shown a typical mask 10 known in the art and used to print the conductive lines on the green sheet blanks. Mask 10 comprises line patterns 12 and 13 each having a rectangular base section 14 with a width wl connected a to circular xe2x80x9ccapxe2x80x9d section 16 with a diameter dc. Circular cap section 16 is intended to be aligned with a hole punched in the blank, such that the paste flows down into the hole, creating a conductive via. Line pattern 12 has a xe2x80x9cjoggedxe2x80x9d end 15 that is at an angle, typically 90xc2x0 or 135xc2x0, to rectangular base section 14.
Referring now to FIG. 2, there is shown a cross section of a line-to-via connection 20 on a green sheet blank 22 as manufactured using a mask of the prior art such as mask 10 shown in FIG. 1. One known problem, which may be encountered in the creation of such MLC substrates, is that the mask may be misaligned with blank 22 such that the hole punched in the blank for the via 27 is not perfectly centered underneath cap section 16 on mask 10 (shown in FIG. 1). In such case, line-to-via connection 20 between conductive line 24 and conductive via 27 may comprise a necked region 28 within the cap 29.
Cap diameter dc is typically greater than via diameter dv and greater than width wl. For example, line 24 may have a width (wl in FIG. 1) of about 0.071 mm (2.8 mils), via 27 may have a diameter dv of about 0.089 to about 0.1 mm (about 3.5 to about 4 mil s), and cap 29 may have a diameter dc of about 0.114 to about 0.127 mm (about 4.5 to about 5 mils), whereas the alignment capabilities of the mask to the blank may only provide alignment of the cap to the via within an accuracy of about 0.05 mm (2 mils). Necked region 28 comprises an area having a thickness t2 that is less than the thickness t1 of the remainder of line 24. This thinner area is subject to concentrated thermal fatigue stresses during normal operation of the chip, and may be susceptible to cracking, causing a major reliability problem for the MLC package.
To achieve this and other objects, and in view of its purposes, the present invention provides a method for minimizing formation of cracks at junctions between conductive vias and conductive lines in line-to-via connections on a substrate. Each line has a base section and a cap. Each cap is positioned over a via in the substrate, the cap diameter being greater than the via diameter.
The method comprises screen printing the conductive lines onto the substrate with conductive paste using a mask that provides a transition zone connected between the base section and the cap. The transition zone provides a volume of conductive paste during printing that is greater than the volume provided by the base section being directly connected to the cap. In particular, the transition zone volume is an effective amount to prevent necking of the conductive lines into the vias when the mask is misaligned to the substrate within an expected alignment tolerance. In one embodiment, the method comprises providing the transition zone with a greater width than the width of the base section.
The invention thus also comprises an improvement in standard conductive line-to-via connections. A standard conductive line-to-via connection comprises a conductive line connected to a conductive via having a via diameter. The standard conductive line has a base section with a first width and a cap with a cap diameter greater Than the via diameter and greater than the first width. The improvement comprises the conductive line having a transition zone between the base section and the cap, the transition zone having a second width greater than the first width. The transition zone may comprise a jogged end extending from the base section to the cap at an angle to the line, or a flared end extending from the base section.
The invention also comprises a line pattern in a screen printing mask for forming such a conductive line on a substrate. The line comprises a base section, a cap adapted to be positioned over a via in the substrate, and a transition zone connected between the base section and the cap. The line pattern has a cap shape adapted to print the cap, a base section shape adapted to print the base section, and a transition shape connected between the base section and the cap and adapted to print a transition zone connected between the base section and the cap. The cap shape has a cap diameter greater than the via diameter. The transition shape has a width greater than the width of the base section shape. The present invention also comprises a screening mask having such a line pattern.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.